This invention relates to frequency stabilized signal sources for operating at microwave frequencies and more particularly to an improved technique for holding the operating frequency of a microwave signal source within prescribed limits.
It is required that the frequencies of microwave radio transmitters be maintained within specific limits. Although relatively simple automatic frequency control (AFC) circuits are employed to accomplish this function at low frequencies, more complex techniques are required to maintain the frequency of a microwave signal source fixed. A prior art stabilized microwave signal source comprises a microwave oscillator, a reference oscillator unit, and an AFC unit. In the reference oscillator unit, a relatively stable high frequency crystal oscillator provides a 113.5MHz signal, for example, which is amplified and multiplied by 17 up to 1930MHz, and mixed with a 2000MHz radio frequency (RF) signal from the microwave oscillator to produce a 70MHz intermediate frequency (IF) signal. This 70MHz signal contains the frequency variation related to the frequency drift of the microwave oscillator as well as that of the 113.5MHz reference crystal oscillator. This IF signal is amplified and divided down to a low frequency signal of about 5MHz. The divider may be similar to commercially available Motorola MC 1697P emitter coupled logic (MECL) divider units which operate at relatively low frequencies. Alternatively, the divider circuit could be of the type illustrated in the receiving system in U.S. Pat. No. 2,356,201, issued Aug. 22, 1944, for Frequency Modulation Signal Receiving System by G. L. Beers, and the associated article in Proceedings of the IRE, December 1944, page 730-737. A crystal oscillator of the AFC unit produces a very stable low frequency signal of 5MHz which is compared with the divided down 5MHz signal to produce a DC error voltage which is proportial to the frequency drift of the microwave oscillator. This error voltage is applied to the latter oscillator to shift its operating frequency back to 2000MHz. The 113.5MHz reference crystal oscillator and associated power amplifier, multiplier and microwave mixer of the reference oscillator unit are relatively expensive and difficult to tune up for the desired operation. An object of this invention is the provision of a stabilized microwave signal source which does not require a reference oscillator unit.